Fuel cell stack

ABSTRACT

A fuel cell stack is provided in which a plurality of single cells each including a membrane electrode assembly are stacked in a stacking direction. The fuel cell stack includes a plurality of electrical insulation members each connected to an outer peripheral portion of a corresponding one of the membrane electrode assemblies. The fuel cell stack further includes a first displacement absorbing member disposed between each insulation member and an adjacent insulation member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2008-295450 filed Nov. 19, 2008, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a fuel cell stack.

2. Description of the Related Art

A fuel cell, which directly converts chemical energy to electric energyby utilizing electrochemical reaction of reaction gases including ananode gas such as hydrogen and a cathode gas such as oxygen, has beenwell known.

Japanese Unexamined Patent Application Publication No. 2006-92924discloses a solid polymer electrolyte fuel cell stack including aplurality of single cells. Each of the single cells includes a membraneelectrode assembly (hereinafter referred to as “MEA”) and separatorsdisposed on both sides of the MEA. The MEA has an anode electrode and acathode electrode sandwiching an electrolyte membrane therebetween. Inthe outer periphery of the fuel cell stack, insulating resin members areformed so that the stacked single cells can be joined to each other andinsulation from the outside can be ensured.

However, in the fuel cell stack described in Japanese Unexamined PatentApplication Publication No. 2006-92924, the plurality of single cellsand the resin members are integrally formed. Therefore, when theelectrolyte membranes of the MEAs swell and the fuel cell stack expandsin the direction in which the single cells are stacked (hereinafterreferred to as “the stacking direction”), the resin members cannotfollow displacement of the fuel cell stack, or in other words,displacements between the plurality of MEAs. This may cause resinmembers to crack. If the resin members crack, water vapor generated inthe MEAs may leak out from the inside of the fuel cell stack to theoutside, a liquid junction may be generated, and insulation performanceof the fuel cell stack may deteriorate.

SUMMARY OF THE INVENTION

The present invention provides a fuel cell stack in which insulationperformance is restrained from deteriorating.

In one embodiment, a fuel cell stack is provided in which a plurality ofsingle cells each including a membrane electrode assembly are stacked ina stacking direction. The fuel cell stack includes a plurality ofinsulation members each connected to an outer peripheral portion of acorresponding one of the membrane electrode assemblies. The plurality ofinsulation members are electrically insulating. The fuel cell stackfurther includes a first displacement absorbing member disposed betweeneach insulation member and an adjacent insulation member.

In another embodiment, a fuel cell stack is provided in which aplurality of single cells are stacked in a stacking direction. The fuelcell includes a plurality of membrane electrode assemblies eachincluding an electrolyte membrane and outer peripheral membersconfigured and arranged to absorb displacement between the plurality ofmembrane electrode assemblies. Each of the outer peripheral members isconnected to an outer peripheral portion of a corresponding one of theplurality of membrane electrode assemblies.

In another embodiment, a fuel cell stack is provided including aplurality of membrane electrode assemblies and displacement absorbingmeans for absorbing displacements between each of the membrane electrodeassembly and an adjacent membrane electrode assembly and for supportingthe plurality of membrane electrode assemblies at outer peripheralportions thereof.

When the electrolyte membranes of the membrane electrode assembliesswell, the first displacement absorbing members, the outer peripheralmembers, or the displacement absorbing means deform so that theinsulation members can follow displacement of the fuel cell stack in thestacking direction, whereby the insulation members are prevented fromcracking.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate preferred embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain features of theinvention.

FIG. 1 is a schematic view of a fuel cell stack of a first embodiment;

FIG. 2 is a partial sectional view of adjacent single cells in thestacking direction;

FIGS. 3A and 3B illustrate the relationship between the thickness of asingle cell in the stacking direction and the thickness of a protrudingportion of an insulation member in the stacking direction;

FIGS. 4A and 4B illustrate the sealing ability of the fuel cell stackagainst water vapor generated in an MEA;

FIGS. 5A to 5E illustrate states of insulation members when the fuelcell stack expands in the stacking direction;

FIG. 6 is a partial sectional view of single cells of a fuel cell stackof a second embodiment in the stacking direction; and

FIG. 7 is a partial sectional view of single cells of a fuel cell stackof a third embodiment in the stacking direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below with referenceto the drawings.

First Embodiment

A fuel cell system directly converts chemical energy of fuel to electricenergy. In a fuel cell system, an electrolyte membrane is sandwichedbetween an anode electrode and a cathode electrode. The anode electrodeis supplied with anode gas including hydrogen, and the cathode electrodeis supplied with cathode gas including oxygen. The followingelectrochemical reactions occur on the surfaces of the anode electrodeand the cathode electrode in contact with the electrolyte membrane, sothat electric energy is obtained from the electrodes.

Anode electrode reaction: 2H₂→4H⁺+4e ⁻  (1)

Cathode electrode reaction: 4H⁺+4e ⁻+O₂→2H₂O  (2)

FIG. 1 shows a fuel cell stack 100, which is a fuel cell system used fora mobile vehicle such as an automobile.

The fuel cell stack 100 includes a plurality of single cells 10, a pairof collector plates 20, a pair of insulation plates 30, a pair of endplates 40, and nuts 50 screwed into tension rods (not shown).

The single cells 10, which generate electromotive force, are unit cellsof a solid polymer electrolyte membrane fuel cell type. The fuel cellstack 100 includes a stack of the single cells 10. The structure of thesingle cell 10 is described below in detail with reference to FIG. 2.

Each of the pair of collector plates 20 is disposed on an outer surfaceof the stack of the single cells 10. The collector plates 20 are made ofgas-impermeable electroconductive material such as compact carbon. Eachof the collector plates 20 has an output terminal 21 on an upper sidethereof. The fuel cell stack 100 outputs electrons generated in thesingle cells 10 through the output terminals 21.

Each of the pair of insulation plates 30 is disposed on an outer surfaceof a corresponding one of the collector plates 20. The insulation plates30 are made of insulating rubber.

Each of the pair of end plates 40 is disposed on an outer surface of acorresponding one of the insulation plates 30. The end plates 40 aremade of metal or resin material having rigidity. One of the end plates40 includes a cooling water inlet 41A, a cooling water outlet 41B, ananode gas inlet 42A, an anode gas outlet 42B, a cathode gas inlet 43A,and a cathode gas outlet 43B.

The nuts 50 are disposed on outer surfaces of the pair of end plates 40at positions near the four corners of each of the end plates 40. Thenuts 50 are screwed into ends of each of the tension rods extendingthrough the fuel cell stack 100. The fuel cell stack 100 is fastened inthe stacking direction by the tension rods and the nuts 50. In order toprevent a short-circuit between the single cells 10, the surfaces of thetension rods are insulated.

Alternatively, the fuel cell stack 100 may be fastened in the stackingdirection by using tension plates.

Referring to FIG. 2, the structure of the single cell 10 is described.FIG. 2 is a partial sectional view of adjacent single cells 10 in thestacking direction. Each of the single cells 10 includes an MEA 60, ananode separator 71 and a cathode separator 72 sandwiching the MEA 60therebetween, and an insulation member 80 integrally formed with the MEA60.

The MEA 60 is a layered stack including an electrolyte membrane 61, ananode electrode 62 disposed on one surface of the electrolyte membrane61, and a cathode electrode 63 disposed on the other surface of theelectrolyte membrane 61.

The electrolyte membrane 61 is a proton-conductive ion exchange membranemade of fluorocarbon resin. The electrolyte membrane 61 is larger thanthe anode electrode 62 and the cathode electrode 63, so that theelectrolyte membrane 61 has an outer edge 61A which extends past theouter edges of the anode electrode 62 and the cathode electrode 63.Because the electrolyte membrane 61 conducts electricity well in a wetcondition, the anode gas and the cathode gas are humidified in the fuelcell stack 100.

The anode electrode 62 is a stack of layers including an electrodecatalyst layer made of an alloy including platinum or the like, awater-repellent layer made of fluorocarbon resin or the like, and a gasdiffusion layer made of a carbon cloth or the like, which are stacked onthe electrolyte membrane 61 in this order.

As with the anode electrode 62, the cathode electrode 63 is a stack oflayers including an electrode catalyst layer, a water-repellent layer,and a gas diffusion layer, which are stacked on the electrolyte membrane61 in this order.

The anode separator 71 is a corrugated panel of electroconductivematerial such as metal. The anode separator 71 is larger than the MEA60. On the side of the anode separator 71 which contacts the anodeelectrode 62, an anode gas passage 71A for supplying anode gas to theanode electrode 62 is formed between the anode separator 71 and theanode electrode 62. On the opposite side of the anode separator 71, acooling water channel 71B, through which cooling water for cooling thefuel cell stack 100 flows, is formed between the anode separator 71 andthe cathode separator 72.

The cathode separator 72 is a corrugated panel made of electroconductivematerial such as metal. The cathode separator 72 is larger than the MEA60. On the side of the cathode separator 72 which contacts the cathodeelectrode 63, a cathode gas passage 72A for supplying the cathode gas tothe cathode electrode 63 is formed between the cathode separator 72 andthe cathode electrode 63. On the opposite side of the cathode separator72, a cooling water channel 72B, through which cooling water for coolingthe fuel cell stack 100 flows, is formed between the cathode separator72 and the anode separator 71.

The cooling water channel 71B, which is formed by the anode separator 71of one of a pair of adjacent single cells 10, and the cooling waterchannel 72B, which is formed by the cathode separator 72 of the otherone of the pair of adjacent single cells 10, face each other. Thecooling water channels 71B and 72B constitute a cooling water channel73.

The insulation member 80, which is made of electrically insulatingresin, is a frame-shaped member disposed along the outer periphery ofthe MEA 60. The insulation member 80 includes a frame portion 81integrally formed with the outer periphery of the MEA 60, and aprotruding portion 82 protruding from the frame portion 81 in thestacking direction.

The protruding portions 82 of the insulation member 80 jut out from anend of the frame portion 81 both ways in the stacking direction(vertical directions in FIG. 2). The protruding portion 82 of theinsulation member 80 of one of a pair of adjacent single cells 10 andthe protruding portion 82 of the insulation member 80 of the other oneof the pair of single cells 10 are bonded to each other via a firstdisplacement absorbing member 90. In one embodiment, the firstdisplacement absorbing members 90 are bonding members.

In the frame portion 81 of the insulation member 80, a slot 83 is formedso that the outer edge 61A of the electrolyte membrane 61 can beinserted therein. The frame portion 81 is sandwiched between the anodeseparator 71 and the cathode separator 72 of the single cell 10, andbonded to the anode separator 71 and the cathode separator 72 via seconddisplacement absorbing members 92. In one embodiment, the seconddisplacement absorbing member 92 are bonding members.

The first displacement absorbing members 90, with which the spacebetween the insulation members 80 is filled, and the second displacementabsorbing members 92, with which the insulation member 80 is bonded tothe separators 71 and 72, can be adhesives having a Young's moduluslower than that of the insulation member 80 when the adhesives arecured. It is preferable that the Young's modulus of the firstdisplacement absorbing members 90 and the second displacement absorbingmembers 92 is equal to or lower than 20 MPa.

Since the fuel cell stack 100 includes the insulation members 80covering the outer peripheries of the single cells 10, insulationbetween the inside and the outside of the fuel cell stack 100 can beensured.

As shown in FIG. 3A, the thickness t1 of the protruding portion 82 ofthe insulation member 80 of the single cell 10 in the stacking directionis smaller than the thickness t2 of the single cell 10 in the stackingdirection. The thickness t2 of the single cell 10 in the stackingdirection is the sum of the thickness of the MEA 60 in the stackingdirection, the thickness of the anode separator 71 in the stackingdirection, and the thickness of the cathode separator 72 in the stackingdirection. If, for example, the thickness t1 of the protruding portion82 of the insulation member 80 in the stacking direction is larger thanthe thickness t2 of the single cell 10 in the stacking direction asshown in FIG. 3B, the insulation members 80 of adjacent single cells 10interfere with each other, so that the contact pressure between thesingle cells 10 decreases, which may impair power generation efficiency.In the present embodiment, the thickness t1 of the protruding portion 82in the stacking direction is smaller than the thickness t2 of the singlecell 10 in the stacking direction. Therefore, the insulation members 80of adjacent single cells 10 do not interfere with each other, wherebypower generation efficiency is less likely to be impaired.

FIGS. 4A and 4B illustrate the sealing ability of a fuel cell stackagainst water vapor generated in an MEA. FIG. 4A shows the fuel cellstack 100 of the present embodiment, and FIG. 4B shows a fuel cell stack200 of a comparative example.

In the fuel cell stack 200 shown in FIG. 4B, insulation members 80 aredisposed so as to sandwich an outer edge 61A of an electrolyte membrane61 of a single cell 10 therebetween, whereby the outer periphery of anelectrolyte membrane 61 is exposed to the outside. Thus, as shown by thearrow A, water vapor generated in an MEA 60 can be easily released tothe outside from between the electrolyte membrane 61 and the insulationmembers 80. Moreover, in the fuel cell stack 200, rubber gaskets 74 aredisposed between the insulation members 80 and the separators 71 and 72.Because the rubber gaskets 74 are permeable to water vapor, water vaporgenerated in the MEA 60 may leak to the outside from between theinsulation members 80 and the separators 71 and 72 as shown by the arrowB.

In contrast, in the fuel cell stack 100 of the present embodiment shownin FIG. 4A, the outer edge 61A of the electrolyte membrane 61 of thesingle cell 10 is inserted into the slot 83 of the frame portion 81 ofthe insulation member 80, and the frame portion 81 is bonded to theseparators 71 and 72 via the second displacement absorption members 92,whereby water vapor generated in the MEA 60 is restrained from passingbetween the insulation member 80 and the separators 71 and 72. Moreover,in the fuel cell stack 100, the space between the protruding portions 82of the insulation members 80 of adjacent single cells 10 is filled withthe first absorption displacement member 90, so that the inside of thefuel cell stack 100 is separated from the outside by the insulationmembers 80 and the first displacement absorbing members 90. Thus, evenif water vapor passes between the insulation member 80 and theseparators 71 and 72, the water vapor is prevented from leaking to theoutside.

When the fuel cell generates power, the electrolyte membranes 61 of theMEAs 60 swell, so that the fuel cell stack 100 expands in the stackingdirection. FIGS. 5A to 5E illustrate states of an insulation member whenthe fuel cell stack expands in the stacking direction.

FIG. 5C shows a fuel cell stack 300, which is a comparative example ofthe fuel cell stack 100. In the fuel cell stack 300, protruding portions82 of insulation members 80 of adjacent single cells 10 are connected toeach other via a first displacement absorbing member 90 having a Young'smodulus higher than that of the insulation members 80. FIG. 5D shows afuel cell stack 300, which is a comparative example of the fuel cellstack 100. In the fuel cell stack 300, protruding portions 82 ofinsulation members 80 of single cells 10 are integrally formed with eachother. With the fuel cell stack 300 shown in FIGS. 5C and 5D, theinsulation members 80 and the first displacement absorbing members 90may not be able to follow displacement of the fuel cell stack(displacements between a plurality of MEAs 60) in the stackingdirection, which may occur during power generation or on otheroccasions. Causes of displacement include, but are not limited to,swelling of the MEAs 60 and vibration of the fuel cell stack 300 forexample in a moving automotive vehicle subjected to unevenness in theroad. Therefore, the protruding portion 82 of the insulation member 80,for example, may crack as shown in FIG. 5E.

In contrast, in the fuel cell stack 100 shown in FIG. 5A, the protrudingportions 82 of the insulation members 80 of adjacent single cells 10 arebonded to each other via the first displacement absorbing member 90having a lower Young's modulus than the insulation members 80. As shownin FIG. 5B, the first displacement absorbing member 90 deforms so thatthe insulation member 80 can follow displacement of the fuel cell stackin the stacking direction, whereby the insulation member 80 isrestrained from cracking.

Accordingly, the fuel cell stack 100 of the present embodiment has thefollowing advantages.

In the fuel cell stack 100, the space between the protruding portions 82of the insulation members 80 of adjacent single cells 10 is filled withthe first displacement absorbing member 90. Even when the electrolytemembrane 61 of the MEA 60 swells, the first displacement absorbingmember 90 deforms so that the insulation member 80 can follow thedisplacement of the fuel cell in the stacking direction, whereby theinsulation member 80 is restrained from cracking. Therefore, water vaporgenerated in the fuel cell stack 100 does not leak to the outside andgeneration of a liquid junction is suppressed, whereby the insulationperformance of the fuel cell stack 100 is restrained from deteriorating.

In the fuel cell stack 100, the frame portion 81 of the insulationmember 80 is integrally formed with the outer periphery of the MEA 60.Thus, as compared with the fuel cell stack 200 shown in FIG. 4B, inwhich the outer edge 61A of the electrolyte membrane 61 is sandwichedbetween the insulation members 80, the area of the electrolyte membrane61 that swells can be decreased. Therefore, with the fuel cell stack100, the displacement of the fuel cell stack in the stacking directiondue to swelling of the electrolyte membrane 61 can be reduced ascompared with the fuel cell stack 200, whereby the insulation member 80is more securely restrained from cracking.

In the fuel cell stack 100, the space between the protruding portions 82of the insulation members 80 of adjacent single cells 10 is filled withthe first displacement absorbing member 90, so that the inside of thefuel cell stack 100 is separated from the outside. Thus, water vaporgenerated in the MEA 60 does not leak out of the fuel cell stack 100.Moreover, in the fuel cell stack 100, the outer edge 61A of theelectrolyte membrane 61 of the single cell 10 is inserted into the slot83 of the frame portion 81 of the insulation member 80, and the frameportion 81 of the insulation member 80 is bonded to the separators 71and 72 via the second displacement absorbing member 92. Thus, watervapor generated in the MEA 60 is restrained from passing between theinsulation member 80 and the separators 71 and 72. In this manner, watervapor is restrained from leaking to the outside without using gaskets orthe like, whereby the number of components and the size of the fuel cellstack 100 can be reduced.

Second Embodiment

FIG. 6 is a partial sectional view of single cells 10 of a fuel cellstack 100 of a second embodiment in the stacking direction.

The fuel cell stack 100 of the second embodiment, which is similar tothat of the first embodiment, differs from that of the first embodimentin that the stacked state of the single cells 10 is firmly held in thesecond embodiment. The difference is mainly described below.

The fuel cell stack 100 includes a stack of the single cells 10 eachincluding an insulation member 80, and the stack of the single cells aresandwiched between end plates 40 in the stacking direction. Thus, therigidity of the fuel cell stack 100 in a direction perpendicular to thestacking direction is lower than the rigidity of the fuel cell stack 100in the stacking direction. Therefore, when a force is applied to thefuel cell stack 100 from the outside in a direction perpendicular to thestacking direction, the single cells 10 may be moved in the directionperpendicular to the stacking direction. If the single cells 10 aremoved by a large distance, the insulation member 80 and the firstdisplacement absorbing member 90 may not be able to follow the movementof the single cells 10, whereby the insulation member 80 and the firstdisplacement absorbing member 90 may crack.

In order to prevent this, as shown in FIG. 6, the fuel cell stack 100includes a pair of tie rods 84 so that the single cell 10 can berestrained from moving.

The tie rods 84 extend in the stacking direction along the outerperipheral surfaces of the protruding portions 82 of the insulationmembers 80 of the stack of the single cells 10. The tie rods 84 arefixed to the end plates 40. The pair of tie rods 84 are disposed so asto face each other and sandwich the single cells 10 therebetween fromouter sides of the insulation members 80.

The fuel cell stack 100 of the second embodiment includes the pair tierods 84, which are disposed on the outer sides of the stack of thesingle cells 10 and extend in the stacking direction. Thus, even when aforce is applied from the outside in a direction perpendicular to thestacking direction, the single cells 10 are restrained from moving inthe direction perpendicular to the stacking direction. Since the singlecells 10 are restrained from moving in the direction perpendicular tothe stacking direction, the first displacement absorbing member 90 candeform so that the insulation members 80 can follow movement of thesingle cells 10. Thus, the insulation members 80 and the firstdisplacement absorbing members 90 are restrained from cracking.Therefore, water vapor generated in the fuel cell stack 100 does notleak to the outside, generation of a liquid junction is suppressed, andthe insulation performance of the fuel cell stack 100 is restrained fromdeteriorating.

Third Embodiment

FIG. 7 is a partial sectional view of single cells 10 of a fuel cellstack 100 of a third embodiment in the stacking direction.

The fuel cell stack 100 of the third embodiment, which is similar tothat of the first embodiment, differs from that of the first embodimentin the structure of insulation members 80 of the single cells 10. Thedifference is mainly described below.

In the third embodiment, a protruding portion 82 of the insulationmember 80 of the single cell 10 protrudes from an end of the frameportion 81 one way in the stacking direction (upward in FIG. 7). Inorder to absorb displacement of the fuel cell stack, a space between theprotruding portion 82 of the insulation member 80 of one of a pair ofadjacent single cells 10 and an end of the frame portion 81 of theinsulation member 80 of the other one of the pair of adjacent singlecells 10 is filled with a first displacement absorbing member 90.

Also with the fuel cell stack 100 of the third embodiment, the firstdisplacement absorbing member 90 deforms so that the insulation member80 can follow the displacement of the fuel cell stack in the stackingdirection, whereby the advantage similar to that of the first embodimentcan be gained.

Moreover, in the fuel cell stack 100 of the third embodiment, the firstdisplacement absorbing member 90, which deforms in accordance with thedisplacement of the fuel cell stack, is disposed farther from ends ofthe anode separator 71 and the cathode separator 72 in the stackingdirection than in the first embodiment. Therefore, when the firstdisplacement absorbing member 90 deforms so that the insulation member80 can follow the displacement of the fuel cell stack, the insulationmember 80 and the ends of the separators 71 and 72 are prevented fromcolliding with or shifting with respect to each other, whereby theinsulation member 80 is restrained from cracking.

While the invention has been disclosed with reference to certainpreferred embodiments, numerous modifications, alterations, and changesto the described embodiments are possible without departing from thesphere and scope of the invention, as defined in the appended claims andequivalents thereof.

For example, the advantages of the above-described embodiments can beobtained by using displacement absorbing means such as an elastic membermade from rubber or other like material instead of the bonding member asthe first displacement absorbing 90. Moreover, in each of the first tothird embodiments, the fuel cell stack 100 is made by simultaneouslystacking all the single cells 10. However, some of the single cells 10may be stacked beforehand to form a cell module, and a plurality of cellmodules may be stacked so as to form the fuel cell stack 100. With thisstructure, the number of man-hours required for assembling the fuel cellstack can be reduced.

Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but that it have the full scope defined by thelanguage of the following claims.

1. A fuel cell stack in which a plurality of single cells each includinga membrane electrode assembly are stacked in a stacking direction, thefuel cell stack comprising: a plurality of insulation members eachconnected to an outer peripheral portion of a corresponding one of themembrane electrode assemblies, the plurality of insulation members beingelectrically insulating; and a first displacement absorbing memberdisposed between each insulation member and an adjacent insulationmember.
 2. The fuel cell stack according to claim 1, wherein the Young'smodulus of the first displacement absorbing members is lower than theYoung's modulus of the insulation members.
 3. The fuel cell stackaccording to claim 1, wherein the first displacement absorbing member isa bonding member bonding adjacent insulation members to each other. 4.The fuel cell stack according to claim 1, wherein each of the insulationmembers includes a frame portion formed along an outer periphery of themembrane electrode assembly and a protruding portion protruding from theframe portion in the stacking direction.
 5. The fuel cell stackaccording to claim 4, wherein the protruding portion protrudes from theframe portion both ways in the stacking direction.
 6. The fuel cellstack according to claim 4, wherein the protruding portion protrudesfrom the frame portion only one way in the stacking direction.
 7. Thefuel cell stack according to claim 4, further comprising: a pair ofseparators sandwiching the membrane electrode assembly therebetween,wherein the thickness of the protruding portion in the stackingdirection is smaller than the sum of the thickness of the membraneelectrode assembly in the stacking direction and the thickness of thepair of separators in the stacking direction.
 8. The fuel cell stackaccording to claim 4, wherein the membrane electrode assembly includesan electrolyte membrane, and wherein the frame portion defines a slotinto which an outer edge of the electrolyte membrane is inserted.
 9. Thefuel cell stack according to claim 4, further comprising: a pair ofseparators sandwiching the membrane electrode assembly therebetween; andsecond displacement absorption members each disposed between the frameportion and the separators.
 10. The fuel cell stack according to claim9, wherein the Young's modulus of the second displacement absorbingmembers is lower than the Young's modulus of the insulation members. 11.The fuel cell stack according to claim 10, wherein the seconddisplacement absorbing members are bonding members bonding the frameportion to the separators.
 12. The fuel cell stack according to claim 1,further comprising a pair of tie rods extending in the stackingdirection and sandwiching the single cells therebetween from outer sidesof the insulation members.
 13. A fuel cell stack in which a plurality ofsingle cells are stacked in a stacking direction, the fuel cellcomprising: a plurality of membrane electrode assemblies each includingan electrolyte membrane; and outer peripheral members configured andarranged to absorb displacement between the plurality of membraneelectrode assemblies, each of the outer peripheral members beingconnected to an outer peripheral portion of a corresponding one of theplurality of membrane electrode assemblies.
 14. The fuel cell stackaccording to claim 13, wherein each of the outer peripheral membersincludes an insulation member that is electrically insulating and adisplacement absorption member that absorbs the displacement, theinsulation member being connected to the outer peripheral portion of thecorresponding one of the plurality of membrane electrode assemblies, thedisplacement absorption member being disposed between the insulationmembers of adjacent outer peripheral members.
 15. The fuel cell stackaccording to claim 14, wherein the Young's modulus of the displacementabsorption member is lower than the Young's modulus of the insulationmember.
 16. The fuel cell stack according to claim 14, wherein thedisplacement absorption member is a bonding member bonding theinsulation members to each other.
 17. The fuel cell stack according toclaim 14, wherein each of the insulation members includes a frameportion formed along an outer periphery of the membrane electrodeassembly and a protruding portion protruding from the frame portion inthe stacking direction.
 18. A fuel cell stack comprising: a plurality ofmembrane electrode assemblies; and displacement absorbing means forabsorbing displacements between each of the membrane electrode assemblyand an adjacent membrane electrode assembly, and for supporting theplurality of membrane electrode assemblies at outer peripheral portionsthereof.